Recrystallization-derived Porous V₂O₅ Polyhedrons For Superior Aqueous Zinc Batteries

The aqueous zinc batteries are regarded as the most promising alternatives for lead-acid batteries owing to low cost and safety,and are expected to dominate the next generation grid-energy storage devices market.It is substantial to research on cathode materials which possess high capacity,high power density and long lifespan.The ideal cathode materials for aqueous zinc batteries must be equipped with structure capable of ingressed/egressed Zn²⁺reversibly,and working voltage window exluding in hydergen evolution and oxygen evolution reaction.The high capacity(589 m A h g^-1)and non-aggressive redox potential(0.65/0.5 V、1.0/0.9 V vs.Zn/Zn²⁺)enable vanadium pentoxide（V₂O₅）to be the preferred cathode material for aqueous zinc batteries.The frustrating issues in V₂O₅ come from the high cost,sluggish ion diffusivity,and deteriorating cycling capacity.What’s more,the storage mechanism is also under debate.In this thesis,the recrystallization method along with soft-template is used to synthesis hierarchical porous V₂O₅ with different morphologies,which is later screened by cell parameters and electrochemical performance.And then,the focus research of phase transition under different current density in electrode process is conducted in order to determine the optimal activation current density.On the basis of above,the electrochemical performances between different woring voltage window are scrutinized.The main contents and results are as following:（1）Developing hierarchical porous V₂O₅via recrystallization method.The raw materials,ammonium metavanadate（AM）powder,are recrystallized under different ratio of polyvinylpyrrolidone-DI water-ethanol system to be precursors,which are then heated to become hierarchical porous V₂O₅materials.In this method,the（001）and（110）lattice plane in the obtained orthorhombic phase V₂O₅grow preferentially,which are in favor of diffusion of Zn²⁺in interlayer.Especially,the one（No.3-2 V₂O₅）having the most strong peak ratio of I（001）/I（301）and I（110）/I（301）express the most outstanding electrochemical performance:the 70.6%capacity retention after cycled 2000 times under 2 A g^-1,274 m A h g^-1achieved under 10 A g^-1 and the diffusion coefficient up to 1.84×10^-12cm² S^-1.Benefit from hierarchical porous structure,the need for“quick charging”is satisfactory for the high contribution of pseudo-capacitance.（2）Phase transition during the V₂O₅electrode process and the influence of current density in activation process.The research on mechanism is achieved through ex-situ XRD and ex-situ XPS tests on the electrodes dis-/charged to certain voltage in the first cycle,by using the No.3-2 V₂O₅ in previous.When the cell is activated under 0.2 A g^-1,the crystal structure is maintained well,in addition,the side reaction hydrogen evolution reaction is also restrained.In terms of the mechanism,Zn²⁺and H⁺co-insert in V₂O₅,while OH^-reversibly precipitate/dissolute in forms of Zn₄SO₄（OH）₆·5H₂O and Zn₃（OH）₂V₂O₇·2H₂O,which is reversible along dis-/charge process.（3）The electrochemical performance of Zn//V₂O₅battery under different working voltages.The suitable cut-off voltage is confirmed to be 0.2 V through the cycling and rate capability.Further,the cycled batteries are characterized by the Hybrid Pulse Power Characterization（HPPC）to acquire the quantified parameters of discharge/charge resistance and pulse power capability.When the working voltage window is set to 0.2-1.6 V,the side reaction hydrogen evolution reaction and polarization are both restrained.In addition,after 2000 times cycling,the cell still has comparative low resistance and large pulse power ability.